Variable inductance



May 25, 1948. M L 2,442,264.

' VARIABLE INDUCTANGE.

Filed Aug. 22, 1944 15 5 7 15 1 11 \fiyygi. I

Z INVENTOR.

Planted nagua I. v 2,442,264 V UNITED STATES, PATENT J orncs Richard L. Campbell, Haywood, N. 3., aaslznorto Allen B. Du Mont Laboratories, Inc., Pascale,

N. 1., a corporation of Delaware Application Allglllt 22, 1944, No. 550,567

2 Claims. (Cl. 171-242) p 1 2 This invention relates to an inductance coil of A cylindrically shaped body or tube 8 of inwhich the inductance can be varied very easily sulating material surrounds the cylinders 3 and and can be adjusted very accurately to the desired 4 and fits tightly around them. This may be a amount. strip 01' impregnated paper or other suitable in- In carrying out the invention, atube of insulat- '5 sulating material wound or wrapped into a coil ing material has a coil of wire wound upon it with the layers adhered together and mounted and this tube is slidably adjustable along and on the cylinders 3 and 4 to provide a form upon beyond a core of magnetic material onto a core which the coil 1 of wire is wound. of non-magnetic material so as to vary the in- The coil 1 of copper or other insulated fine wire duc'tance. These cores are supported by metal '10 of conducting material is wound on the tube 6, rods that serve as conducting members to which the length of this wire being suiilcient to provide the ends of the coil are attached. the desired'amount of variable inductance. The

This inductance coil is particularly useful as amount of inductance is determined by the posia compensating coil for wide band amplifiers. tions of the tube 6. on the cylindrical bodies 4 In a wide band amplifier the residual or stray l5 and 5.

capacities are often a limiting factor. By this A strip I of thin insulating material such as invention an extremely small yet emcient com- Celluloid or mica is wound around the coil 1 and pensating coil is provided in which the stray cathe convolutions are stuck togetherto provide a.

pacity introduced into the circuit is very small. cylinder tightly fitting this coil.

Another object of the invention is to provide One end of the coil 1' is connected to a flexible means that can be easily and quickly adjusted lead Ill consisting of several strands of fine flexii'or providing the correct compensation in amble wire twisted together and being longer than plifier stages, using vacuum tubes. the axial length of the coil 1. The other end oi Another object of the invention is to make use the lead 10 is attached'to the terminal I by solof the permeabilityeilects of high frequency 'iron dering, as indicated at H. The other end of the in order to reduce the amount of turns of wire coil I is attached in a similar manner by the lead that is required for a compensating coil, with a R to the terminal 2 as indicated at I3 so that consequent saving in physical size and decrease in the coil 1 will not become detached from the tercapac tance minais I and 2 as this coil isadjusted for varying Another object is to minimize distributing ca- 0 the inductance. Loops i 4 and 14' are provided pacitance and provide inexpensive means for in the connecting leads II and I2 to facilitate varying the inductance. adjustment of the coil 1 longitudinally on the The invention may be understood from the decylinders 3 and 4 to vary the inductance.

s pti n in nn t on th t e pm ns In the modification shown in Fig. 3 the coil 1' drawing, in wh is' of much shorter axial length than the coil 1 Fig. 1 is a longitudinal section of an illustrative and t is of mu h larger external diameter.

embodiment of the invention; A strip' I! of insulating material is laid longi- 2 is a section along the line H of s- 1; 'tudinally oi the tube 6 before the coil 1 is installed. Fig. 3 is a section through a modification; and After the coil 1 d cylinder I are in place the Figs 4 5 are sideviews of other modmca 40 extending end of this strip I5 is then. folded Hons back and attached to the other end thereof by 5 3? g gg ifiia fig gg i i g: cement or other adhesive material, thus forming made f nommagnetic material such as copperan extending tab it by means of which the coil for example. The ends of these terminals i and 46 1 is moved or adjusted along cylinders and 5 2 are enlarged as shown at and and anchored to vary the inductance.

in cylinders 3 and 4 of insulating material. The- 1* Wm be obvious from the disclosure that when cylinder 3 is made 01' rigid insulating material, the central transverse plane of the coil 1 is in such as Bakelite, and the cylinder 4 is made of line with the central transverse plane of the cylrigid insulating material in which very m ll or 50 inder 4 .the inductance will be a maximum, and microscopic segregated particles of powered iron t y m vin this 1 by means of the a are embedded. The cylinders 3 and s ar nto the limit onto the cylinder 3 the inductance nected to the enlarged ends of the terminals I will be a m The ndu ta ce of -18 and 2 so as to be rigidly held in aligned fixed made in accordance with this invention depend position with respect to each other. 66 upon the number of turns of the wire in coil I and the dimensions of the parts used and the position of the coil 1.

The following is given as a specific illustrative example of a variable inductance coil made in accordance with Fig. 1 of this invention:

Fifty-six turns of 40E copper wire were wound at the rate of 260 turns per inch as a single layer coil 0.20 inch long upon a cylindrical form of insulating material 0.375 inch long. Measurements showed that the inductance of this coil ranged from 15 to 30 microhenrys and the capacitance thereof was 0.6 of a micro-microfarad In the modification shown in Fig. 4 reference characters 2| and 22 are terminal rods or supports attached respectively to the cylinder 23 of rigid insulating material such as a phenolic compound and cylinder 24 of magnetic material such as powdered iron and a binder. A brass disc 25 is attached to the outer end of cylinder 23. The coil 21 with tab 28 is similar to the coil 1 in Fig. 1 and is mounted and moved in a way similar to the coil 1. The ends of the coil 21 are connected by flexible leads 29 and 30 to the rods 2| and 22.

In this modification the dimension and proportions are such that when the coil 21 is centered on the iron core 24 the inductance is a maximum: when it is centered on the phenolic cylinder 23 the inductance is half the maximum; and when it is centered on the brass disc 25 the inductance is one-fifth the maximum.

In the modification shown in Fig. 5, reference characters 3| and 32 are conducting supports for the solid cylinders 33 and 34. The adjacent ends of these cylinders are attached together, for example, by cementing them together. The cylinder 33 is made up of particles of conductin material 33' having some resistance, such asv carbon and a binder, and an insulating shell 33". The cylinder 34 is similar to cylinder 4, Fig. 1, and is therefore magnetic. The disc 35 attached to the outer end of cylinder 33 is metal, such as brass or copper, and is in conducting contact with this cylinder. The coil 36 is made up in a way similar to the wire coil 1 with a rigid or stifi insulating shell inside and outside of the wire coil and is provided 'with a tab 38 for adjusting it longitudinally. Flexible leads 39 and 33' connect the ends of coil 36 to the supports ill and 32. A ring 43 provided with an attach- 4 ment lug ll fits around the insulating shell 33" but is removable.

In an embodiment of the invention which has proven to be satisfactory, when the coil 33 is centered upon the core 35 the inductance is a maximum, when it is centered on the cylinder 33 the inductance is half the maximum, and when it is centered upon the disc 35 it is one-fifth of the maximum.

When the leads 33 and 33' are connected as shown in Fig. 5 the coil 36 and resistance 33' are in parallel between the conducting supports 3| and 32. When the lead 39 is disconnected from the ring 40 the terminal 3| and lead 39 can be connected to a source of current so that the coil 36 and resistance 33' are in series.

What is claimed is:

1. An inductance, comprising a support consisting of two aligned spaced metal rods, a cylinder of insulated particles of magnetic material rigidly mounted on one of said rods, a cylinder of insulating material of the same diameter as said first named cylinder mounted on the other one of said rods, a cylindrical strip of adherent insulating material wound upon adjacent ends of said cylinders and holding them in alignment and a coil of insulated wire slidable on said cylindrical strip, said coil being substantially the length of said first named cylinder and being movable longitudinally of said cylinders and having its ends connected respectively to said metal rods.

2. The device of claim 1 in which the adjacent ends of said cylinders are in contact.

RICHARD 'L. CAMPBELL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,913,544 Haben June 13, 1933 2,066,945 Putnam Jan. 5, 1937 2,097,642 Putnam Nov. 2, 1937 2,221,217 Kirk et al Nov. 12, 1940 2,236,292 Kirk Mar. 25, 1941 2,285,128 Rosenberg, Jr June 2, 1942 2,285,378 Patzschke June 2, 1942 

